For my money, the wooden cask is one of mankind’s greatest inventions—the gift that keeps on giving.
Not only does it hold booze—or less interesting things, if you’re so inclined—but over time it works its divine chemical wonders in tandem with Mother Nature and Father Time to mold wine, beer, or spirit into something far greater than the sum of its parts.
Some spirits simply cannot exist without the cask. Most whiskey would be a (literally) pale parody of itself without cask influence. Cognac, Calvados, and Armagnac? Unaged examples are floating around, but they practically scream for a barrel’s warm embrace.
However, despite the cask’s supreme importance to the quality and continuation of our industry, very few folks really understand what’s happening to the spirit inside. And, while you could argue that viewing cask maturation through the lens of chemistry takes away some of the romantic and alchemical fun, I’d wager that a clearer view of things will help you make more educated decisions and produce higher-quality products.
So, with that in mind, let’s take a few moments to consider why the cask is so important, and let’s look at some of the reactions taking place inside.
The Wood’s Magic
To understand what’s happening inside the cask, we first need to understand the characteristics of the liquid we’re putting in there. Most new-make spirit (NMS) directly off the still isn’t exactly ready for prime time; it can often be hot, sometimes headsy or tailsy, and may even have some sulfurous funk to it. Unless it’s been distilled to be released as a white spirit, NMS is usually not that appetizing.
A few years ago, I was doing some work in Scotland with a distiller’s trade organization, and I took the opportunity to visit some of my favorite malt whisky distilleries—including Macallan, one of the quintessential Speyside malts, masterfully matured in bourbon and sherry casks and undeniably complex. At the end of the tour, our group sat down for a quick tasting of the core Macallan line, including, surprisingly, a taste of their NMS diluted to 40 percent ABV.
As a distiller, I could recognize the inherent wonders lurking beneath its feint-heavy surface, but most consumers would be baffled by it. Macallan is designed to be matured for long periods of time in wooden casks. If the company wanted to release the spirit as a new-make, it would most certainly be distilled differently.
Wood—and, more specifically, oak—is a wonderful maturation material for spirits (excluding a few oddball species). It is sturdy but relatively easy to work with and shape into a barrel. Many species of oak such as Quercus alba (American white oak), Q. robur (French oak), and Q. mongolica var. crispula (Mizunara oak) contain a myriad of chemical compounds that lend complexity to the final spirit. Those compounds increase in abundance when the wood is properly treated with heat through toasting and/or charring. And, finally, wood is a porous material that still manages to maintain a liquid-tight seal. That means that vapors can leave the cask while oxygen can enter, all without having to stress much about leaking liquids.
The chemistry behind barrel maturation is bizarre, labyrinthian, and—let’s be honest—poorly understood. While the past 100 years or so have given us a few research projects elucidating some of maturation’s mysteries, a dense conceptual fog still surrounds the whole process and exactly what’s happening.
For my money, the wooden cask is one of mankind’s greatest inventions—the gift that keeps on giving.
Not only does it hold booze—or less interesting things, if you’re so inclined—but over time it works its divine chemical wonders in tandem with Mother Nature and Father Time to mold wine, beer, or spirit into something far greater than the sum of its parts.
Some spirits simply cannot exist without the cask. Most whiskey would be a (literally) pale parody of itself without cask influence. Cognac, Calvados, and Armagnac? Unaged examples are floating around, but they practically scream for a barrel’s warm embrace.
However, despite the cask’s supreme importance to the quality and continuation of our industry, very few folks really understand what’s happening to the spirit inside. And, while you could argue that viewing cask maturation through the lens of chemistry takes away some of the romantic and alchemical fun, I’d wager that a clearer view of things will help you make more educated decisions and produce higher-quality products.
So, with that in mind, let’s take a few moments to consider why the cask is so important, and let’s look at some of the reactions taking place inside.
The Wood’s Magic
To understand what’s happening inside the cask, we first need to understand the characteristics of the liquid we’re putting in there. Most new-make spirit (NMS) directly off the still isn’t exactly ready for prime time; it can often be hot, sometimes headsy or tailsy, and may even have some sulfurous funk to it. Unless it’s been distilled to be released as a white spirit, NMS is usually not that appetizing.
A few years ago, I was doing some work in Scotland with a distiller’s trade organization, and I took the opportunity to visit some of my favorite malt whisky distilleries—including Macallan, one of the quintessential Speyside malts, masterfully matured in bourbon and sherry casks and undeniably complex. At the end of the tour, our group sat down for a quick tasting of the core Macallan line, including, surprisingly, a taste of their NMS diluted to 40 percent ABV.
As a distiller, I could recognize the inherent wonders lurking beneath its feint-heavy surface, but most consumers would be baffled by it. Macallan is designed to be matured for long periods of time in wooden casks. If the company wanted to release the spirit as a new-make, it would most certainly be distilled differently.
Wood—and, more specifically, oak—is a wonderful maturation material for spirits (excluding a few oddball species). It is sturdy but relatively easy to work with and shape into a barrel. Many species of oak such as Quercus alba (American white oak), Q. robur (French oak), and Q. mongolica var. crispula (Mizunara oak) contain a myriad of chemical compounds that lend complexity to the final spirit. Those compounds increase in abundance when the wood is properly treated with heat through toasting and/or charring. And, finally, wood is a porous material that still manages to maintain a liquid-tight seal. That means that vapors can leave the cask while oxygen can enter, all without having to stress much about leaking liquids.
The chemistry behind barrel maturation is bizarre, labyrinthian, and—let’s be honest—poorly understood. While the past 100 years or so have given us a few research projects elucidating some of maturation’s mysteries, a dense conceptual fog still surrounds the whole process and exactly what’s happening.
[PAYWALL]
What we can say is that, in general, we can break barrel maturation processes into four different reaction categories: additive, productive, subtractive, and reductive.
The Wood Giveth …
Additive reactions are exactly what they sound like. These are “reactions” in which compounds from the barrel directly infuse into the spirit. Perhaps we should toss the word “reaction” aside; these compounds get into the spirit through a simple extraction process, not unlike steeping tea leaves in hot water for a few minutes. The compounds in question include things such as wood sugars, tannins, color compounds, and lactones.
Many factors impact additive processes, including the proof of the spirit going into the barrel and the humidity and temperature inside the warehouse. Lower entry strengths tend to favor faster extractions of water-soluble compounds, such as wood sugars and hemicelluloses (which also increase color). Higher strengths tend to favor more alcohol-soluble compounds, such as the lactones responsible for coconut and sawn-wood aromas. Further complicating matters is that higher warehouse temperatures speed up the extraction processes. Humidity also comes into play with evaporation quality: Higher humidity tends to reduce barrel alcohol concentration, and vice versa, changing extraction affinities over time.
The next group are the productive reactions. These are true chemical reactions, which require certain substrate and chemical conditions to occur. Specifically, productive reactions occur when the combinations of various compounds produce maturation congeners. These reactions include esterification, involving a variety of alcohols and organic acids. Ethyl acetate formation immediately comes to mind, but that is far from the only one.
One of the most important precursors to aromatically active compounds coming from the barrel is lignin. Composed of a deceptively simple duo of compounds—coniferyl and sinapyl alcohol—lignin is an incredibly complex polymer. Its purpose in the oak tree is to provide rigidity to cell walls, creating the tree’s woody structure. In barrel maturation, it breaks down into aromatic compounds with relative ease.
Some of this lignin breakdown occurs during the toasting of the cask, forming a variety of important aromatics that include vanillin. During maturation, when lignin comes into contact with ethanol, it can break down through ethanolysis and oxidation, creating a cascade of reactions leading to an array of aromas. For example, coniferyl alcohol can oxidize into coniferaldehyde (bready, graham cracker) which can then oxidize into ferulic acid. Ferulic acid can convert to vanillin—and this cascade of reactions actually extends even further. A similar cascade occurs with sinapyl alcohol.
… and the Wood Taketh Away
If additive and productive reactions contribute more compounds to our spirit matrix, then we shouldn’t be surprised that there are reactions that take things away. The first category we’ll consider is the subtractive reaction set.
Think back to what I said earlier about the initial character of new-make. It can be a headsy, tailsy, sulfur-laden mess. Add all the color, sugar, and lignin-derived goodness you want—it won’t matter unless the aging process removes some aspects of the NMS. Otherwise, you’ll just be left with a rough-and-tumble spirit fit for no more than cleaning your floors.
Subtractive reactions come in a few different flavors, so to speak. The first is the adsorption of compounds onto the barrel’s char layer, which is nothing more than activated carbon. We use activated carbon in an assortment of situations throughout our daily lives, such as the carbon water filters attached to home faucets. As it turns out, the char layer in a barrel does a nice job of removing a number of unwanted aromas and aromatic baddies.
The classic example of this is in the production of single-malt whiskey. Many distillers opt for a lightly kilned distiller’s malt as their base fermentable in single-malt production. A lot of world-class whiskey is made from this malt, but a lot of it has decent amounts of the sulfur-bearing compound S-methylmethionine (SMM). In higher-kilned malts, the heat easily destroys SMM, but it stubbornly remains in many lightly kilned distiller’s malts. It can break down into dimethyl sulfide (DMS), which smells like cooked corn. Beer brewers can easily remove DMS with an extended boil of the wort. Because distillers don’t typically boil their wort, DMS is a potential problem … until the spirit hits the cask, that is. Over time—often several years—the char layer adsorbs the DMS, making it much less of an aromatic nuisance.
Now, you might be saying to yourself, “I make rum/brandy/agave/[insert non-whiskey fermentable here], so what do I care about char layers?” But DMS and related compounds aren’t the only sulfur-causing culprits in new-make.
Many rum and brandy yeast strains can and will produce hydrogen sulfide (H2S) precursors, often in the form sulfur dioxide. If these precursors go on to form H2S—and they often do—you can have some ugly aromas on your hands. H2S in small amounts smells of burnt matches; in greater amounts it evokes rotten eggs. Much of it will evaporate out of the cask over time, but the char layer will also handle some of it. That’s one reason why it’s important to think about the type of cask you’re using and to pair it appropriately with the spirit you’re making. If you intend to mature your spirit in uncharred casks, one consideration would be to use a no- or low-sulfur-producing yeast.
Another component of the subtractive reaction set is simple evaporation. Acetaldehyde is a natural by-product of fermentation. In small amounts, it contributes green apple–like aromas. In larger amounts, however, it becomes a serious flaw and often contributes to the initial NMS aromas. Fortunately, acetaldehyde has a very low boiling point—about 68°F (20°C)—and it readily evaporates from the cask over time. (You’d think that with such a low boiling point, acetaldehyde would fully wind up in the heads fraction during batch distillation, but because of complicated molecular machinery such as the van der Waals force, this just isn’t the case.)
The final reaction category of note is reductive reactions. Oppositely analogous to the productive reaction category, reductive reactions occur when various reaction pathways reduce the compounds of NMS in both amount and effectiveness. The compound that serves as the poster child for the reductive reaction group is acrolein.
Acrolein is a compound with a peppery, pungent horseradish-like character, and it is found to some degree in most NMS. Acrolein forms naturally from the heat-induced degradation of glycerol inside the still. (It can also form from lactic bacterial contaminations, creating some real headaches for the distiller.) Acrolein is part of the reason why so much NMS tastes a bit hot and “fiery” right off the still.
Fortunately, the cask once again saves the day. As oxygen ingresses into the cask, acrolein is readily oxidizable to the far less impactful acrylic acid. (The “reductive” reaction category is a bit of a misnomer in this case because we’re technically talking about an oxidation reaction and not reduction. Here, reduction really refers to quantity and not which molecules are losing or gaining electrons.)
Applying the Science in the Distillery
Aside from better understanding the inner workings of the barrel and its effect on your NMS, hopefully you can now make more informed decisions in your production processes. I’ve already mentioned an example regarding sulfur-producing yeast strains, but I’m not stingy. Allow me to offer one more.
When we were installing our three-chamber still—the “3C,” as we call it—here at Iron City Distilling, we knew we were in for a different kind of NMS than we were used to with our pot still. The 3C operates at higher pressures—hence, higher boiling points—than our pot still and produces a very feint-heavy spirit. It’s ugly when it comes off the still, and it’s something only a parent could love. However, this rough-and-tumble spirit evolves into something beautiful over time, if given a little TLC.
Immediately, we knew cask selection was going to be important. Here we had a very funky new-make that needed ample maturation time, and perhaps a few things stripped out to let the good stuff shine. So, we opted to use only char No. 4 American oak, placing all these barrels in the highest racks of our steam-heated warehouse, to maximize the chemical interaction rates.
In your own distillery, there are many variables at play when building out a cask management strategy. Start with an honest internal discussion about exactly what you want the final spirit to taste like, then you can decide the optimal cask and warehouse strategies to get you there. Some casks are better suited to build certain spirit types. For example, don’t go thinking that you can make a French-style grape brandy by using ex-bourbon barrels. For one, the bourbon flavors will be out of place. And two, the use of American oak instead of French will yield vastly different results than the style you’re after. The result won’t necessarily be bad, per se, but it won’t be to spec. And that’s all before we get into the questions of warehouse type, environment, and air flow.
Or, what if you want to release an unaged version of your NMS? That means you’ll be taking the barrel out of the equation—and that, too, is fraught with some challenges. Can you simply bottle some of your standard NMS, diluted off the still, while the rest heads toward the barrel warehouse for casking? Sure, you can—many of the big American whiskey distillers have done the same, but those products aren’t exactly flying off the shelves (for multiple reasons, to be fair).
One issue is that those products are distilled to mature inside a barrel, with congeneric complexity built in, so that the cask can work its magic. In many instances, if you want to release an unaged version of your aged spirit, a cleaner distillation procedure is in order. Cut the heads to hearts later and cut a bit earlier when going from hearts to tails. Think of ways to increase the internal reflux of the still.
Trust the Process
If the preceding paragraphs have given you a few heart palpitations, relax. Pour yourself a glass of your favorite tipple and breathe.
It is absolutely true that the cask contributes the bulk of aroma and flavor to the final spirit in aged liquors. So, of course, cask selection, management, and a solid understanding of the underlying chemistry are vital. However, the most important thing you can do to ensure an outstanding final product is to make sure that the liquid going into the cask is the best you can make it.
Use only the finest fermentables and treat them with the greatest respect through fermentation and distillation. Take care of those items, and I can assure you a well-made cask will take care of the rest—regardless of cask chemistry’s complexity.
